1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, an in-plane switching mode liquid crystal display device and a fabrication method thereof that provide a fast response speed and an improved transmittance.
2. Discussion of the Related Art
As the information-oriented age is advancing, display devices for disposing and displaying information are actively being developed. More particularly, a flat panel display device, e.g., a liquid crystal display (LCD) device, having a small thickness, lightness weight and a low power consumption, has been actively studied. An LCD device uses optical anisotropy and birefringence characteristics of liquid crystal molecules in a liquid crystal layer to display images.
An LCD device may be an active matrix type liquid crystal display device having a thin film transistor and a pixel electrode connected to the thin film transistor arranged in a matrix manner. For example, an LCD device includes an upper substrate, a lower substrate, and a liquid crystal layer disposed between the upper and lower substrates. The upper substrate is referred to as a color filer substrate, and a lower substrate is referred to as an array substrate. When a driving voltage is supplied to a common electrode on the upper substrate and a pixel electrode on the lower substrate, a perpendicular electric field is formed between the common electrode and the pixel electrode. Because the liquid crystal molecules in the liquid crystal layer are thin and long, and have a pretilt angle, the pretilt angle is changed by the electric field. Thus, an arranging direction of the liquid crystal molecules is changed, thereby altering the optical anisotropic of the liquid crystal molecules and displaying images.
However, when the liquid crystal layer is driven by the perpendicular electric field, a transmittance and an aperture ratio increase but a viewing angle decreases. Accordingly, to solve this disadvantage, a driving method of liquid crystal by in-plane switching (IPS) using a horizontal electric field has been suggested.
FIG. 1 is a cross-sectional view illustrating an in-plane switching mode liquid crystal display device according to the related art. In FIG. 1, a liquid crystal panel of an in-plane switching mode liquid crystal display device includes a color filter substrate 9 having a color filter, an array substrate 10 having a thin film transistor facing the color filter substrate 9, and a liquid crystal layer 11 disposed between the color filter substrate 9 and the array substrate 10. A common electrode 17 and a pixel electrode 30 are disposed in parallel to each other on the array substrate 10, and a horizontal electric field L is formed by a difference in voltages supplied to the common electrode 17 and the pixel electrode 30. Thus, the in-plane switching mode liquid crystal display device is driven by using the horizontal electric field L to control liquid crystal molecules.
FIGS. 2A and 2B are cross-sectional views illustrating ‘off’ and ‘on’ states of the in-plane switching mode liquid crystal display device shown in FIG. 1. As shown in FIG. 2A, in an ‘off’ state, when no voltages are supplied to the common electrode 17 and the pixel electrode 30, no horizontal electric field is formed. Thus, all liquid crystal molecules 11 are aligned in the same direction.
As shown in FIG. 2B, in an ‘on’ state, when voltages are supplied to the common electrode 17 and the pixel electrode 30, the horizontal electric field L is formed. Locations of liquid crystal molecules 11a that are located corresponding to the common electrode 17 and the pixel electrode 30 remain unchanged by the horizontal electric field L. However, liquid crystal molecules 11b that are located between the common electrode 17 and the pixel electrode 30 become aligned in the same direction as the horizontal electric field L. Accordingly, the in-plane switching mode liquid crystal display device has a broad viewing angle, because the liquid crystal moves by the horizontal electric field. As a result, the in-plane switching mode liquid crystal display device may be viewed in direction of above/below/left/right of about 80° to 85° without a reversal process.
However, since the common electrode and the pixel electrode are formed on the same substrate in a pixel region, they shield the pixel region, thereby decreasing an aperture ratio. In addition, since a light quantity passing through the liquid crystal display device is limited, a brightness decreases.
In addition, the general liquid crystal display device (TN mode liquid crystal display device) and the in-plane switching mode liquid crystal display device use a twisted nematic liquid crystal. Since the twisted nematic liquid crystal has a response time over 30 ms (i.e., a slow response speed), the liquid crystal display device using a twisted nematic liquid crystal has a problem of low display quality in that an afterimage occurs when implementing an animation or fast movements. To improve these problem of the response speed, an FLC mode liquid crystal display device using a ferroelectric liquid crystal having a superior response property has been developed.
The ferroelectric liquid crystal is often referred to as a chiral smectic C liquid crystal having a response time below many ms. In other words, a response speed of the ferroelectric liquid crystal molecules is fast. In particular, each layer of the chiral smectic C liquid crystal is arranged with an angle. When an electric field is supplied to the chiral smectic C liquid crystal, a dipole moment is arranged in one direction, and a molecular alignment is uniform and maintained after the electric field is eliminated. Further, when an electric field is supplied in an opposite direction of the chiral smectic C liquid crystal, the molecular alignment may be reversed in an opposite direction at high speed. Thus, the molecular alignment of the ferroelectric liquid crystal differs according to a polarization of an electric field, and the FLC mode liquid crystal display device has a fast response property.
FIG. 3 is a cross-sectional view illustrating an FLC mode liquid crystal display device using a ferroelectric liquid crystal according to the related art. In FIG. 3, a liquid crystal panel 40 of an FLC mode liquid crystal display device includes a ferroelectric liquid crystal layer 80 in a gap d1 between a first alignment layer 55 on an array substrate 50 and a second alignment layer 75 on a color filter substrate 70. The ferroelectric liquid crystal layer 80 includes a plurality of ferroelectric liquid crystal molecules 82 arranged with an angle in the gap d1. The gap d1 is generally smaller than 2 μm.
Thus, the FLC mode liquid crystal display device has problems in that since the gap between the substrates is below 2 μm and since the ferroelectric liquid crystal molecules are in almost a gel state at a normal temperature, it is difficult to inject the ferroelectric liquid crystal molecules in the gap. In addition, because the gap is small, it is also difficult to provide a wider degree of control of the ferroelectric liquid crystal molecules using a supplied electric field.